Rechargeable power supplies generally consist of a number of rechargeable batteries or cells arranged in series, in parallel or in a series/parallel combination. It is common to provide Zener diodes connected in parallel across each cell to prevent the recharging voltage supplied to the cell from exceeding a predetermined level, which could result in damage to the cell, and to allow the power supply to continue to supply power to any connected equipment in the event of one or more of the cells failing in an open circuit condition.
In such an arrangement, the Zener diodes act as voltage regulators whereby if the voltage applied to the cell exceeds a certain level, break down of the Zener diode will occur causing current to leak through the Zener diode in the reverse direction. As the applied voltage increases the current leaking through the Zener diode also increases. This has the effect of clamping the voltage over the cell to a predetermined level.
To ensure that the cell is not subjected to a voltage higher than its maximum rating, the Zener diode used must be capable of leaking a reverse current which is at least equal to the charging current of the power supply at a voltage which is less than or equal to the cell's maximum voltage rating. However, Zener diodes generally do not have a stepped on-off break down characteristic and thus permit leakage current through the diode, to greater or lesser extent, over a range of voltages. Thus, a Zener diode having a 2.5 mA leakage current at a breakover voltage of 3.3 V may still permit 50% of that current to pass through at an applied reverse-biased voltage of 3.2 volts. Consequently, once the power supply is fully charged and the charging current removed, the leakage current through the Zener diodes will continue until the voltage over the cell has been reduced to a point where the Zener diode has zero leakage current. During this time, the leakage current through the Zener diode will cause discharging of the cell.
Since rechargeable cells are effective for only a limited number of charge/discharge cycles, this number usually being a function of the percentage depth of each cycle, the discharge effect of the Zener diode on the cell means that the potential life of the cell, and therefore power supply as a whole, is significantly reduced, both in terms of each charge/discharge cycle and the overall life of the power supply. This is a particular problem in low power applications (for example smoke detectors/alarms), where the leakage current through the Zener diodes is often a high multiple of the supply current required to power the equipment. This has the effect of reducing the life of the rechargeable power supply to a fraction of its potential life.